This invention relates to the synthesis of substituted borazines, the thermal polymerization of these borazines, and the final transformation of the preceramic borazine polymers into carbon-free boron nitride ceramic.
Ceramic materials, of which boron nitride is representative, have unique properties of great commercial importance. However, due to their insolubility and nonfusibility, the processing of these materials into useful end-products presents serious difficulties. Bulk boron nitride is commonly prepared by fusion of borax with ammonium chloride (Tied, H. et al., Z. Anorg. Allgem. Chem. 147, p. 114 (1925)) or fusion of boric anhydride with urea in an ammonia atmosphere (O'Connor, T., J. Am. Chem. Soc. 84, p. 1753 (1952)). However, because of its nonfusible and insoluble ceramic nature, the bulk material is not useful in applying boron nitride as a coating to complex surfaces or forming it into complex shapes. For these purposes, it is necessary to generate boron nitride in situ from soluble, processible precursors. A critical requirement for such precursors is that the boron nitride they yield on fusion should be free of carbon, a troublesome impurity in ceramics.
Borazines are potentially suitable as this type of boron nitride precursor because of their ability to form soluble polymers on heating. Preceramic polymers based on borazines where no carbon is bonded to either boron or nitrogen are most suitable for the production of pure boron nitride fibers and coatings.
The literature reports the polymerization of a few substituted borazines; but the mechanism of reaction as well as the chemical structure and composition of the polymers is not clear, so it is not always possible to predict the structure or composition of the product of a synthesis. Gerrard, W. et al., J. Chem. Soc., p. 113 (1962), postulated that the interaction of chloroborazine with a class of molecules represented by di-s-butylamine would produce polymerized borazine rings attached through ring boron and nitrogen atoms; however, the polymerization of B-tris (ethylamino)-N-triethyl borazine on heating for four hours at 300.degree. C. was reported to polymerize to borazine rings joined by ethyl-substituted nitrogen atoms.
A method to produce polymers of borazines consisting of a repeated structure of boron-nitrogen bonds and using various alkyl aminoborazines as starting material is the subject of a Japanese patent (Japanese Patent No. 37,837 (1978) to Tanaguchi). However, a careful reproduction of this procedure was found to produce polymers whose pyrolysates were impure boron nitride due to carbon retention (Paciorek, K. et al., J. of Polymer Sci. 24, pp. 173-185 (1986)).
The inventors have produced borazine polymer capable of final transformation into carbon-free boron nitride by the synthesis of triamino-N-tris (trialkylsilyl) borazines and subsequent thermal condensation into preceramic polymer. The composition and method are the subject of U.S. Pat. No. 4,581,468 (1986). This polymer is referred to herein as polymer A.
It is the principal object of the present invention to provide a preceramic borazine polymer having a different structure from the previous patented material, but which is also capable of undergoing a conversion to pure boron nitride upon pyrolysis. The polymer of the present invention is referred to herein as polymer B.
The organic solvent-soluble polymers A and B can be used as precursors to permit the use of boron nitride ceramic in applications otherwise impractical or impossible. After the preceramic polymer is appropriately placed, it can be transformed under reactive conditions, which remove substituent groups from the polymerized borazine rings, into a residue of pure boron nitride.
A readily processible polymer which upon pyrolysis can be so transformed into a pure boron nitride ceramic offers a potential for producing the ceramic in many useful physical forms, including fibers, shaped articles, coatings, foams, and also as a binder for ceramic powders which would eliminate the use of additives (i.e., sintering aids).
It is another principal object of this invention, therefore, to provide a suitable preceramic polymer which is soluble in organic solvents, and which can be formed into solvent spun fibers or coatings.
It is also an object of this invention to provide processes for transforming the preceramic polymer into fibers, coatings and shaped articles of pure boron nitride ceramic.
Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure.